AFAIK it is impossible -- as shamanNS said, Amazon implemented the Goodreads functionality using the ASIN.
In fact, a number of features are enabled by using calibre-created ebooks with a special embedded ASIN... cover thumbnails, appearing as an EBOK (instead of PDOC), etc...
But all that precludes using Amazon's whispersync, available only through the Personal Documents service... which automatically undoes any and all other modifications you have done, and creates an unidentified PDOC.
It's all Amazon's fault -- it's like they want to subtly encourage you to buy the book from them.
